WebSocket for live updates in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

WebSocket server message broadcast output

What will be the output on the client console when the server broadcasts a message to all connected clients?

Raspberry Pi

import asyncio
import websockets

connected = set()

async def handler(websocket):
    connected.add(websocket)
    try:
        async for message in websocket:
            for conn in connected:
                if conn != websocket:
                    await conn.send(f"Broadcast: {message}")
    finally:
        connected.remove(websocket)

async def main():
    async with websockets.serve(handler, "localhost", 8765):
        await asyncio.Future()  # run forever

# Client code snippet:
# async def client():
#     async with websockets.connect("ws://localhost:8765") as ws:
#         await ws.send("Hello")
#         response = await ws.recv()
#         print(response)

# Assume two clients connect and one sends "Hello"

A No output

B Broadcast: Hello

C Error: Cannot send message to self

D Hello

💡 Hint

Think about how the server sends messages to all clients except the sender.

The server sends the message to all connected clients except the one who sent it. So the other client receives 'Broadcast: Hello'.

🧠 Conceptual

intermediate

WebSocket connection states

Which WebSocket connection state indicates that the connection is open and ready to communicate?

A CONNECTING

B CLOSING

C CLOSED

D OPEN

💡 Hint

Think about when you can send and receive messages.

The OPEN state means the connection is established and ready for data transfer.

🔧 Debug

advanced

Identify the error in WebSocket server code

What error will this WebSocket server code raise when run?

Raspberry Pi

import asyncio
import websockets

async def handler(websocket):
    async for message in websocket:
        await websocket.send(message.upper())

async def main():
    server = await websockets.serve(handler, 'localhost', 8765)
    await server

asyncio.run(main())

A TypeError: 'WebSocketServer' object is not awaitable

B RuntimeError: Event loop is closed

C SyntaxError: invalid syntax

D No error, runs correctly

💡 Hint

Check how the server object is awaited.

The 'websockets.serve' returns a server object that is not awaitable. Awaiting it causes a TypeError.

📝 Syntax

advanced

Correct WebSocket client connection syntax

Which option shows the correct way to connect to a WebSocket server and receive a message in Python?

A

with websockets.connect('ws://localhost:8765') as ws:
    message = await ws.recv()
    print(message)

B

async with websockets.connect('ws://localhost:8765') as ws:
    message = ws.recv()
    print(message)

C

async with websockets.connect('ws://localhost:8765') as ws:
    message = await ws.recv()
    print(message)

D

with websockets.connect('ws://localhost:8765') as ws:
    message = ws.recv()
    print(message)

💡 Hint

Remember that 'recv' is an async function and needs 'await' inside an async context.

Option C correctly uses 'async with' and 'await' for the async recv method.

🚀 Application

expert

Number of messages received in a WebSocket client

Given this client code connecting to a server that sends a message every second for 5 seconds, how many messages will the client receive before closing?

Raspberry Pi

import asyncio
import websockets

async def client():
    async with websockets.connect('ws://localhost:8765') as ws:
        count = 0
        try:
            while True:
                msg = await asyncio.wait_for(ws.recv(), timeout=6)
                count += 1
        except asyncio.TimeoutError:
            pass
        print(count)

asyncio.run(client())

A 5

B 6

C 0

D Raises TimeoutError

💡 Hint

The server sends 5 messages, one each second, and client waits up to 6 seconds for each.

The client receives all 5 messages within the timeout and then times out waiting for a 6th message, stopping the loop and printing 5.